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This is by no means a comprehensive
glossary of astronomical terms however it should provide some assistance to
novices in particular. Click on the
symbol for a diagram or picture associated with that
word. Cross references within the glossary are indicated by hyperlink
text. Click on the letters below to go to the first entry of alphabetical
sections.

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Click the link to go to the contents
page (lists all of the words in the dictionary) or click on the letters
below to go to the first entry of each alphabetical section. Click the link
to get back to the site entry page.

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Faculae

These are brighter
patches of the Sun's surface. They are often seen near sunspots.
They are best seen near the edge of the Sun because they contrast better
with the apparently darker limb. They often herald the appearance of sunspots
and may be present after a sunspot has subsided. Some faculae are seen
in high Solar latitudes and are not associated with sunspots
at all. Also associated with faculae are plages or flocculi - clouds that
are high in the chromosphere. These are apparently best observed in light
emitted by ionised calcium.

The moment
at which an eclipse
begins. In the case of a Solar eclipse,
it is the moment that the Moon starts to move across the face of the Sun.
In the case of a Lunar eclipse,
it is when the Moon first enters the shadow of the Earth.

Flare

Flares are created
in the active
regions of the Sun by a sudden release of energy. This accelerates
charges particles in the region which, in turn, causes radiation to be
emitted across the whole spectrum
from X-rays to radio waves. Particles are also ejected from the Sun and
travel through the Solar System. These cause magnetic disturbances on
the Earth. This can cause blackouts and loss of quality in radio communications
and aurorae. If the blast of radiation from the flare is poerful enough,
it can cause systems failures in spacecraft and has been know to shut
down regional power systems on the surface.

Fourth Contact

The moment
at which an eclipse
ends. In the case of a Solar eclipse,
it is the moment that the Moon is seen to move completely off the face
of the Sun. In the case of a Lunar eclipse,
it is when the Moon finally leaves the shadow of the Earth.

Galactic Cluster

This is
an alternative name for an open star cluster in our Galaxy.
It indicates that the cluster lies within our Galaxy rather than in the
halo round our Galaxy, where globular
clusters lie.

Galaxy

A collection of
stars gas and dust. There are many different types but they can be broadly
grouped into 3 main categories - spiral galaxies, elliptical galaxies
and irregular galaxies. When written with a capital 'G', it refers to
our own galaxy, often called "the Milky Way". The Universe is
populated with galaxies. There is very little intergalactic (between galaxies)
matter.

Gegenschein

Gegenschein
is also known as the counterglow.
It is a faint oval patch of light that is very difficult to see, you need
a clear and moonless night. It is at the antisolar point i.e. exactly
opposite the Sun. It is apparently best seen when the ecliptic is at its
highest above the horizon (midwinter from the northern hemisphere and
vice versa for the southern hemisphere). It is not well understood but
is thought to be caused by the scattering of sunlight from dust in the
main plane of the Solar System. Sometimes it can be seen to be joined
to the zodiacal
light by a parallel sided beam of light. I think that this beam is
called the zodiacal band but cannot be 100% sure. I also believe that
it is larger in the tropics than in the temperate zones. Click here
to visit the NASA website and view a picture of the gegenschein.

Giant Star

As stars
approach the end of their lives, the nuclear reactions that power them,
change. This causes the star to greatly increase in size in a big way.
A giant star has a similar mass to our Sun but is much larger and more
luminous. The word giant is usually prefixed with the colour of the star
e.g. red
giant star.

Gibbous

The phase of the
Moon (or planet) between half and full.

Globular Cluster

Unlike open clusters
which are found within our Galaxy,
globular clusters are external to our Galaxy. They are situated within
a spherical volume of space surrounding our Galaxy. A globular cluster
is a huge (tens to hundreds of light
years in diameter) spherical concentration of stars.
The density of stars is much higher than in the spiral arms of the Galaxy.
This high density means that the gravitational forces within a globular
cluster holds the cluster together, against the gravitational force of
the Galaxy. They are amongst the oldest objects in the Universe, 10,000
million years or more in age. They therefore contain stars that are generally
much older than those in the open clusters of the Galaxy. Some globulars
are bright enough to be seen without optical aid e.g. omega Centauri and
47 Tucanae in the southern hemisphere skies. Probably the best known of
the northern hemisphere is M13 in Hercules. This is a fuzzy blob in small
telescopes and binoculars but with a moderate telescope of 180 mm aperture
or more, together with a medium to high magnification individual stars
can be seen.

Gould's Belt

This is a band
of bright young stars that stretches round the sky from Perseus, Taurus
and Orion to Centaurus and Scorpius via Carina. It is thought to be a
spur off the local spiral arm of our Galaxy.

Granulation

In close up,
the surface of the Sun appears to be mottled. This mottling of the surface
is called granulation. It is thought to be caused by convection currents
in the Sun.

Green Flash

This is a very
rare and difficult to observe atmospheric phenomenon. Blink and you miss
it! Literally. The Green Flash lasts only for a few seconds and is best
seen as the Sun sets over the sea when the air is very clear. The last
segment of the setting Sun is seen to turn green, sometimes it is followed
by a green ray rather like a flame.

Greenhouse Effect

This
is the effect of warming the average temperature of a planet by trapping
heat from the Sun in the atmosphere. The only reason that water on Earth
stays in the liquid form is because we have greenhouse gases in our atmosphere.
Without these, the Earth would have cooled below freezing point and life
would not have existed. Important greenhouse gases are, carbon dioxide,
methane and water vapour. CFC's also have a greenhouse effect but are
regarded more as the culprits of destruction of the ozone layer of the
atmosphere. Current problems with global warming are thought to be occurring
because of the burning of fossil fuels (coal, oil and gas) by humans.
There is a major scientific debate about this - some scientists believe
that the current warming is a natural phenomenon - but can we take the
risk? On Venus, where the atmosphere is largely carbon dioxide, a runaway
greenhouse effect has raised temperatures to rather inhospitable levels,
around 400 deg. C.

Hour angle

The time that
has elapsed since an object was on the meridian or the time that will
be needed to wait for an object to reach the meridian.
West of the meridian - hour angle is positive, east it is
negative.

Immersion

The entry of
an astronomical body into the shadow at the time of an eclipse
or the moment that an object is covered during an occultation.

Inferior

A planet or other
body that orbits closer to the Sun than does the Earth. The inferior planets
(Mercury and Venus)
exhibit phases. Inferior can also be used to describe a conjunction
of these two planets.

Infrared

This is radiation
with wavelengths longer that red in the spectrum
but shorter than radio waves. The range of infrared wavelengths is about
700 nanometres to 1mm. Infrared is radiation is also known as heat radiation.
Infrared radiation from the stars and other bodies is absorbed by water
molecules in the atmosphere and so it is best observed by orbiting telescopes.

Interferometer

Interferometers
can be radio or optical. They use the principle of interference of electromagnetic
waves to enable the combination of light or radio waves from two apertures
to be combined. This greatly increases the resolution of an instrument.
The star Capella was resolved into two stars by a team using optical interferometry
whilst many of the worlds radio astronomers link dishes in different countries
to get better pictures of radio galaxies.

Inverse Square Law

The energy
that is received from a source diminishes with distance in accordance
with this law. What it means is that for two identical sources e.g. stars
of the same colour and brightness, if one is twice as far away, it will
appear four times as faint or if it was three times further away it would
be nine times fainter. This law also works with gravity.

Kepler

Johannes Kepler
was born on Dec 27th 1571. At university, he was so inspired by Copernicus'
theory of the Solar System that he chose to devote his life's work to
astronomy. He devised a theory of his own of regular solids alternating
with spheres, to describe how the planets moved. Based on the observations
of Tycho
Brahe in particular, he realised that his theory was flawed since
it could not be used to accurately predict the positions of the planets.
During the 10 years between 1609 and 1619, he formulated his three Laws
of planetary motion. He even attempted to explain how the planets
might move. He imagined some sort of force radiating from the Sun in the
same way that the spokes of a wheel radiate from the hub. These "force
spokes" pushed the planets round. Newton's theory of gravity led
to the correct solution that the planets orbit the Sun because they are
moving forwards at high speed and at the same time, being pulled in towards
the Sun by its gravitation.

Kirkwood Gaps

These are
regions of the asteroid (minor planet) belt in which few asteroids are
found. The reason for this is because of the gravitational influence of
Jupiter. This disturbs the orbits of asteroids whose orbital periods are
exact fractions of that of Jupiter itself.

Lens

A carefully shaped
piece of glas that is used in telescopes to bend rays of light in order
to bring them to a focus and to magnify images. Simple lenses will focus
different colours at different points, leading to a defect called chromatic
aberration. The construction of good quality lenses requires the use
of different types of glass, glued together. .

Light year

The distance
that light travels in a time period of one year, approximately 6,000,000,000,000
miles!

The area
of space around an object influenced by the magnetic field of that object.
The shape of magnetosphere is influenced bu any stellar
wind in the vicinity.

Magnitude

The brightness
of an astronomical object. On a clear, Moonless night in a location away
from any light pollution, the faintest star visible to the naked eye will
be about magnitude 6. In a city, keen eyed observers may spot stars brighter
than about magnitude 3.

The planet that
is closest to the Sun. For detailed information, including images, click
here.

Meridian

An imaginary
line joining the north and south points of the horizon, passing directly
through the zenith. The reference line for a transit.

Meteor

Commonly called
a shooting star. It is a piece of fast moving debris e.g. dust or fragment
of rock that burns up in the Earth's atmosphere because of friction with
the air. A meteor shower is a display of meteors that seem to come from
the same point (more or less) in the sky. They do so because they are
travelling in similar paths through space. They are believed to be debris
from the wake of comets travelling through the Solar System.

Meteorite

If any portion of a meteor survives its passage
through the atmosphere and lands on the surface of the Earth, it is
then called a meteorite. See also achondrite.

It is a piece
of fast moving debris e.g. dust or fragment of rock that has not entered
the Earth's atmosphere but has the potential to do so.

Metonic cycle

Every 6939.6
days (19 years), the same lunar
phase recurs on the same day of the year. This time period is known as
the Metonic cycle. During each cycle, there are exactly 235 lunations.

Minor axis

The shorter
axis of an ellipse.

Moon

A smaller
body orbiting a planet. Also referred to as a natural satellite.

Nebula

A cloud of gas
and dust lying between the stars. Some are bright whilst others are dark.
Bright ones shining because of reflected light are called reflection nebulae.
Bright ones shining because the gas is exited by radiation from nearby
stars are called emission nebulae.
Dark nebulae appear like holes in the starry background because they are
not illuminated by nearby stars.
Nebulae are often the birthplaces of stars.

Newtonian

A type of reflecting
telescope.

NGC

The New General Catalogue
of Nebulae and Clusters of Stars produced by the astronomer Dreyer.

Node

The points at which
the orbit of a planet intersect the ecliptic.
The ascending node is where the planet crosses the ecliptic from S to
N and vice-versa for the descending node.

North Celestial Pole

From any place in
the Northern Hemisphere, there is a point in the northern sky around which
the stars appear to rotate. It is currently near Polaris, the Pole Star.
There is an equivalent point in the southern hemisphere, although there
is no significantly bright star to mark it.

Nova

A star that suddenly
and temporarily increases its brightness to perhaps 100,000 times normal.
There are a variety of mechanisms that cause this.

Nutation

This is a wobble of the Earth's axis.

Objective

The
light collecting part of a telescope i.e. the main mirror or the lens
at the front of a refracting telescope.

Occultation

When
a small astronomical body is hidden from view by a larger one, e.g. the
Moon hiding a star or planet from view as is moves through space, it is
called an occultation rather than an eclipse.

Open Cluster

Open
clusters
are internal to our Galaxy. One particularly well known open cluster is
the Pleiades (also known as the seven sisters) in the constellation of
Taurus. It is said that some observers with exceptionally good eyesight
can se up to 11 members of the cluster. Others were observed by Charles
Messier, who catalogued objects whilst searching for comets. These are
given "M" numbers e.g. M36. The ones that were not catalogued
by Messier appear in other catalogues of deep sky objects such as the
NGC (New General Catalogue). To the naked eye, under dark skies, the brighter
open clusters appear as faint fuzzy patches but in binoculars or telescopes,
their starry nature can be seen.

Opposition

A
planet is said to be in opposition when it is 180 degrees from the Sun
in the sky.

Orbit

The
path of one astronomical body round another, usually elliptical
in shape.

Parallax

The
apparent change in position of a star caused by the annual motion of the
Earth round its orbit. When viewed from one position, a nearby object
will appear in one place relative to the background. If the observer moves
position, the nearby object will appear to be in a slightly different
position. Try it using your thumb. Close one eye and line up your thumb
with something a bit further away. Swap eyes and you will see your thumb
jump to a different position, it will no longer lined up. This is because
the background is further away and so appears to move less (like the trees
at the side of the road seem to rush 'past' but a town in the distance
moves 'past' more slowly). Nearer stars will therefore seem to be in different
positions at different times of the year, when compared to the background
stars.

Parsec

Comes
from the two words "parallax"
and "second". A Parsec is a distance in space, equal to about
3.26 light years. In terms
of angles, a circle is divided into 360 degrees, a degree is divided into
60 minutes and each minute is divided into 60 seconds. A parsec is the
distance that a star would need to be from the Earth in order to give
a parallax of one second of angular movement against the background sky.

Penumbra

The
lighter part of a shadow during an eclipse.
An observer standing in the penumbra of a Solar eclipse
will see a partial eclipse. This word can also mean the lighter part of
a sunspot. See also umbra

Perigee

The
point in the orbit
of an artificial satellite or the Moon that is closest to the Earth. See
also apogee.

The
phase of a moon or planet is the proportion of the sunlit side that is
visible to an observer. From the Earth, the Moon,
Mercury and Venus go through
a complete range of phases from 0% of the surface visible to 100% visible.
The outer planets show only the gibbous phase. The phase of an astronomical
object depends upon the angle between the observer, the Sun and the object
being observed.

Phase angle

The
angle between the object, Sun and observer. A phase angle of 180 degrees
means that the Sun and the object lie in opposite directions from the
observer and therefore the whole of the sunlit side is visible. A phase
angle of 0 degrees means that the Sun and object ar in the same direction,
as seen by the observer thus the unlit side will be presented to the observer,
rendering the object effectively invisible. A phase angle of 90 or 270
degrees means that half of the sunlit side will be visible to the observer
so a half phase will be visible. Between 0 and 90 degrees, the phases
are waxing
crescents.Between 90 and 180 degrees, waxing gibbous, between 180 and
270 the phases are waning
gibbous and greater than 270 degrees - waning crescent.

Planet

A large object, more or less spherical, that shines
only by reflecting light from the star around which it
orbits. Planets of our Solar System are classed as Rocky (Mercury, Venus,
Earth, Mars, Pluto) or Gas Giants (Jupiter, Saturn, Uranus, Neptune).
Several Extrasolar (outside the Solar System) planets have been announced
by various teams from observatories around the world. Many of these
seem to be larger than any in out Solar System and orbit closer to the
parent star. These have been detected by careful observations of wobbles
in a star's motion. See also Kepler.